The present invention relates to a radio receiver apparatus mounted in a vehicle such as a passenger car, wherein the reception sensitivity is greatly increased, and the S/N ratio is greatly improved.
In a conventional car radio receiver apparatus mounted in a passenger car, a reception antenna is externally installed on a metal car body, and a radio receiver is installed in the interior of the car and near the driver's seat. The antenna element is connected to the radio receiver through an antenna signal line.
This radio receiver includes a tuner, a detector, and a low-frequency amplifier. A tuner control knob for controlling the tuner and a volume control knob for controlling the amplifier are mounted on the radio receiver housing. The operating mechanisms such as the tuner control knob and the volume control knob are readily accessible to an operator, including the driver.
FIG. 1 schematically shows the overall arrangement of a conventional car radio receiver apparatus of the type described above. A rod-shaped antenna element 11 extends from an external side portion of a hood of the car body or from an external side portion of a rear trunk thereof. A radio receiver 12 is mounted in, for example, a dashboard disposed inside the car. The radio receiver 12 includes a tuner circuit 13 and an audio signal converter 14, the latter of which provides low-frequency amplification of the high-frequency reception signal detected and tuned in by the tuner circuit 13. A control unit 15 supplies a tuner control signal and a volume control signal to the tuner circuit 13 and to the audio signal converter 14, respectively. The control unit 15, in turn, receives operating signals from an operating section 16. In this case, the operating section 16 is disposed at the front panel of the dashboard where the radio receiver 12 is installed.
The tuner circuit 13 of the radio receiver 12 is connected to the antenna element 11 thereof through a coaxial cable 17 which conducts radio signals. Therefore, a radio signal is received by the antenna element 11, is detected and tuned in by the tuner circuit 12, and an amplified signal is then produced by a speaker 18. In this case, a ground circuit for the coaxial cable 17 and the radio receiver 12 is connected to the metal car body and they are thus grounded.
The tuner circuit 13 of the radio receiver 12 comprises a parallel resonant circuit of an inductor L and a variable diode C.sub.X, as shown in FIG. 2. The control unit 15 causes the capacitance of the diode C.sub.X to vary in accordance with the operation of the operating section 16, so as to tune in to a desired frequency. In this case, the electromotive force of the reception from the antenna element 11 is transmitted to the tuner circuit 13 through the coaxial cable 17. For this reason, a capacitor C.sub.C is connected in parallel with the variable diode C.sub.X. If the capacitor C.sub.C is not present, a tuning frequency f.sub.T of the tuner circuit 13 is given as follows: EQU f.sub.T =1/2.pi..sqroot.LC.sub.X
However, since, as described above, the capacitor C.sub.C is connected in parallel with the diode C.sub.X, the actual tuning frequency f.sub.T is given as follows: EQU f.sub.T =1/2.pi..sqroot.L(C.sub.X +C.sub.C)
The relationships among the inductance of the inductor L, the capacitance C (=C.sub.X +C.sub.C), and the tuning frequency f.sub.T are shown in FIG. 3.
In the car radio receiver apparatus of the type described above, the coaxial cable 17 generally has a length of 1 to 3 m, and the capacitance of the capacitor C.sub.C is considerably large. Therefore, the tuning frequency f.sub.T based on the capacitance of the variable diode C.sub.X has a range between a frequency f.sub.2 where the capacitance of the diode C.sub.X is "0" and a frequency f.sub.1 corresponding to a maximum value of the capacitance of the capacitor C.sub.C by a variation .DELTA.C.sub.X in the capacitance of the diode C.sub.X. Therefore, a desired frequency range cannot correspond to a range between frequencies f.sub.3 and f.sub.4.
Furthermore, since the capacitor C.sub.C is present, the electromotive force E at the antenna element 11 is absorbed as electrostatic energy of "1/2.multidot.C.sub.C .multidot.E.sup.2 " and reaches the tuner circuit 13 of the radio receiver 12. As a result, the input signal component is decreased, and optimal reception sensitivity cannot be obtained. In order to improve the reception sensitivity, it is proposed that a high-frequency preamplifying means be arranged ahead of the radio receiver 12. This preamplifying means must cover the entire frequency range of the radio wave signals to be received and must transmit the signals received by the antenna element to the radio receiver 12. More specifically, the highfrequency preamplifying means must cover a frequency band of 0.5 MHz to 110 MHz, which includes a range between a medium wave broadcast frequency and an ultrashort wave broadcast frequency. Furthermore, the frequency band of the signal to be actually received by the radio receiver of the type described above is limited to a narrow frequency band corresponding to the broadcast frequency fd of a radio station to which the receiver 12 is tuned. However, the high-frequency amplifier amplifies a signal having a frequency band considerably wider than the above-mentioned narrow frequency band.
Now assume that a broadcast radio wave is present which has a frequency close to the frequency fd. The nonlinear characteristics of the frequency preamplifying means together with the action of the frequency fd generate a new frequency component. When the frequency of the new signal is increased to have the same frequency as the frequency fd, a loud noise occurs. In order to eliminate the noise described above, the nonlinear characteristics of the high-frequency preamplifying means must be eliminated by a complex circuit. As a result, the circuit arrangement becomes complicated, and the efficiency of the electrical characteristics is degraded.